The use of a synthetic binary solid
solution to model igneous textural evolution
Elizabeth Scott 2002
A thesis presented to the Faculty of the University at Albany, State
University of New York in partial fulfillment of the requirements
for the
degree of Master of Science
College of Arts & Sciences, Department of Earth and Atmospheric
Sciences
Advisor: J.G. Arnason

Abstract
Crystallization experiments using a synthetic igneous analog have
shown
that grain boundary migration can take place in melt-present
environments
(Means and Park, 1994). In order to evaluate the importance of
metamorphic
processes during the textural evolution of a crystal mush,
microstructural
evidence is needed to link them with final igneous textures. In
natural
magmatic systems, plagioclase may provide a microstructural link;
plagioclase
solid solution enables the process of chemical zoning and
preserves a record
of crystal morphology (and chemistry) during growth. If
metamorphic processes
are common, they should affect the zoning patterns of plagioclase
crystals.
A petrographic stage heater apparatus was designed and built for
the
purpose of observing crystallization processes with a new
plagioclase analog.
The analog utilized synthetic compounds within the (K, NH4)SCN
system
and was crystallized at low temperatures (< 172°C) for
observation
in situ with an optical microscope. The melting points of
compounds containing
XK = (0, 0.25, 0.50, 0.75, 1) were measured in both
thin section
and in sealed capillary tubes in order to construct a phase
diagram. Compounds
of intermediate composition XK = (0.25, 0.50, 0.75)
created
a continuous solid solution with initial and final melting points
between
82°C and 171-175°C in thin section, and between 108°C
and 144°C
(±1) in sealed capillary tubes. The differences between the
two
data sets are probably due to the absorption of atmospheric water
and differing
abilities of the two systems to contain the water.
Textural studies of quenched compounds with composition XK
= 0.8 and XK = 0.75 resulted in the formation of
crystals with
concentric extinction patterns (under cross polarized light) that
resemble
patterns produced by chemical zoning in plagioclase. Grain
boundary migration
was also observed between chemically homogenous grains within the
solid
solution. However, relatively slow rates of cooling were required
to produce
grain boundary migration. While grain boundary migration was only
observed
in unzoned solid solution crystals, further crystallization
experiments
utilizing faster rates of cooling may produce grain boundary
migration
in chemically zoned crystals.